1. Field of the Invention
The present invention relates to a turbomachine including a plurality of blade stages on the trajectory of a flow of air or gas.
2. Description of the Prior Art
In this kind of turbomachine, one or more cavities are provided on the outside of the flow of air and communicate with the flow of air via a plurality of orifices formed in a symmetrical wall along the axis of the turbomachine delimiting the flow of air. A pipe including a discharge valve is generally connected to the cavity to sample a portion of the flow of air to be rejected to the outside when the turbomachine is operating under partial load to improve stable operation of the turbomachine or to satisfy an auxiliary demand. The volume of the cavity must therefore be sufficient to enable regular sampling in use.
It is nevertheless found that the air or the gas flowing across the cavity can trigger acoustical resonance in the cavity in some speed ranges because of boundary layer shear.
Such resonance is encouraged by the structure of the cavity, which is generally symmetrical. The cavity can be symmetrical with respect to the axis of the turbomachine or incorporate patterns, bosses or other raised members regularly distributed over its periphery to produce cyclic symmetry. The break in the symmetry caused by the intake of the pipe for sampling air for the discharge valve or for cooling the discs and blades of the turbine of the turbomachine is insufficient for it to be certain that acoustical resonance in the cavity will be prevented.
Acoustical resonance has major drawbacks and can lead to the risk of blades breaking.
The invention therefore relates to a turbomachine including means for eliminating or preventing the generation of rotating acoustical waves in the previously mentioned cavity and therefore suppressing the drawbacks due to acoustical resonance in said cavity.
The turbomachine in accordance with the invention includes a plurality of blade stages on the trajectory of a flow of air or gas delimited by a symmetrical wall along the axis of the turbomachine, the wall having orifices communicating with a cavity outside the flow of air, of generally axially or cyclically symmetrical structure. Symmetry degrading means are provided inside said cavity.
The symmetry degrading means can take various forms.
In a preferred first embodiment of the invention the symmetry degrading means include a spacer mounted in a pipe connected to the cavity to project partly into the cavity.
The spacer is preferably force-fitted into said pipe to limit vibration in operation.
The spacer can advantageously be a portion of tube force-fitted into said pipe from the outside.
In a second embodiment the symmetry degrading means include a convex localized area of the cavity having its convex side facing towards the inside of the cavity.
The convex area can be obtained by localized stamping of the outside or inside wall of the cavity.
In another embodiment of the invention the symmetry degrading means include a member fixed to the inside face of the outside wall of the cavity at a particular location, for example a portion of sheet metal welded to the inside face of the outside wall of the cavity or to the inside wall of the cavity.
In another embodiment of the invention the symmetry degrading means include a screw passing through the outside wall of the cavity and projecting into the cavity.